The laser diode has been used frequently as a light source in a variety of photographic applications. For example, in the field of laser printing, a laser beam is focused through a lens and then scanned onto a negative film. The diode is often controlled by digital image data stored in a computer. In some systems, the laser is operated so that it is completely on or off, thereby generating either black or white pixels to form an image in response to the image signals. However, image quality may be improved by forming the image from pixels having varying gray tones, more specifically, when gradations in intensities of the gray levels is such that the transitions between pixels appear relatively smooth. This is referred to as a continuous tone image. The continuous tone radiographic laser printer is one application in which a laser diode is used to expose radiographic film to electronic images produced by medical imaging modalities such as computed tomography, magnetic resonance imaging, digital subtraction angiography, ultrasound imaging, and the like.
A problem with laser diodes is that the illuminative output of a semiconductor laser diode is not linear across its entire operating range. More specifically, FIG. 1 is a typical graph of light output as a function of current through the laser diode. The resulting curve has a lower level non-linear operating region joined by a knee region to a higher level operating region where light output varies linearly as a function of the voltage differential (current). The linear region is called the lasing region, and the non-linear, lower level region is known as the light emitting or spontaneous emission region. The curve includes a lower portion between 0 and the knee current I.sub.k where the resulting optical power output P.sub.d varies in a non-linear manner as current I.sub.d changes and a second higher portion between I.sub.k and I maximum current, I.sub.max, where optical power output varies linearly with changes in current I.sub.d. In the non-linear region of the curve below I.sub.k, a constant current increment along the X axis generates a non-constant increment in optical power output along the Y axis. The non-linear region is unsatisfactory for generating a continuous tone image in response to image input signal levels ranging from a zero output level (corresponding to maximum film density) and a maximum signal level (corresponding to minimum film density).
In order to address this problem, commonly assigned U.S. Pat. No. 4,774,710, issued Sept. 27, 1988, entitled APPARATUS AND METHOD FOR CONTROLLING A LASER DIODE TO GENERATE A LINEAR ILLUMINATIVE OUTPUT, discloses the direct modulation of a laser diode in which the diode is amplitude modulated in the linear lasing region and is pulse width modulated in the non-linear light emitting region.